Moving object detection plays an important role in the automatic surveillance systems. The image surveillance system analyzes the trajectory and behavior of moving objects in the surveillance images to detect the abnormal security events and notifies the security for handling the events.
However, the conventional surveillance system usually sets up a plurality of fixed surveillance cameras in the environment under surveillance. When security event occurs, the conventional system may not provide dynamic security support; therefore, the use of mobile camera is gaining popularity. Nevertheless, the mobile camera may cause the entire image change, which may render the conventional moving object detection techniques inapplicable to the detection of security events.
U.S. Pat. No. 6,867,799 disclosed a method and apparatus for object surveillance with a movable camera. The object surveillance apparatus includes modules for moving object detection, interest object manual selector, determining relative difference between object and camera, predicting future position, and generating moving signals. The feature of the disclosed patent is to construct a system capable of moving the camera, based on the user selected moving object area, so that the moving object stays within the visible image frame.
U.S. Pat. No. 5,991,428 disclosed a moving object detection apparatus and method, able to detect foreground moving object in a mobile camera screen image. The feature of the disclosed patent is to divide the image frame into a plurality of blocks by using template matching, and evaluation and voting mechanism to estimate the displacement vector of the corresponding blocks of the camera images captured by successive time spots. Based on the dominant motion vector of the entire image, an alignment vector between neighboring frame images may be determined. Based on the alignment vector, one of the frame images may be shifted for alignment, and the alignment difference comparison can be performed. The comparison result is analyzed to detect the moving area.
U.S. Pat. No. 5,473,364 disclosed a video technique for indicating moving objects from a movable platform. This technique provides a moving object detection system with a movable platform carrying two cameras or a mobile camera. The locations of the two cameras or the mobile camera must be the same as the direction of the vehicle motion. The feature of the disclosed patent is to store an image captured by the front camera at a time into the memory. When the vehicle moves for a small period of time, an image is captured by the rear camera. After aligning the image stored in the memory, and subtracting the image from the rear camera, a Gaussian pyramid construction is used to compute the area energy and detect the moving object.
In the disclosed patent, the assumption is that the image of the front camera at a previous time and the image of the rear camera at a later time have only small difference. After the alignment, the error between the background of the front camera image and the background of the rear camera image is much smaller than the error of the images captured by a fixed camera at two different times and aligned. Therefore, the subtraction can lead to more stable profile of the moving object.
U.S. Pat. No. 6,710,722 disclosed an image processing device and method for sensing moving objects and rangefinder employing the same. The technology is able to detect moving object from a complicated background. The feature of the disclosed patent is to store a series of camera captured images into the memory. The first several images are averaged to serve as the background image, and then subtracting the current image to obtain the moving object. The patent uses two cameras and a three-dimensional visual technique to compute the distance of the moving object.
In the aforementioned and other moving object detection methods, image alignment and three-dimensional visual optical flow tracing are two of the most common moving object detection methods for movable platforms. As shown in FIG. 1, image alignment is to estimate the two-dimensional alignment parameter of two images, that is, one previous image and one posterior image, and then to align one image, for example the previous image, with the other image, for example the posterior image. By subtracting the two images, the difference is obtained and the moving object is detected. In the image alignment technique, the computation is fast and only one camera is required. However, only the difference in object profile may be obtained from this technique. In addition, the error may occur when the background object in the scene is too close to the camera.
As shown in FIG. 2, the three-dimensional visual optical flow tracing method is to obtain first the optical flow trajectory of the two images, a previous image and a posterior image, and combined with depth information to estimate the platform movement parameter. Based on the platform movement parameter and depth information, the background optical flow trajectory of the background may be estimated. By subtracting the optical flow trajectory of the two images, and obtaining the difference, the moving object may be detected. The three-dimensional visual optical flow tracing technique has a small error, and may be used to obtain solid object. However, the disadvantages may include slow computation, and requiring many cameras to obtain the depth information which is difficult to obtain at smooth images.